lib/process: Cleanup stdout callback

lib/process: Cleanup stdout callback

General cleanup of async stdout processing.

The process_stdout_cb and process_stdout_custom routines were doing the
same thing, so rename process_stdout_custom to process_process_stdout
and make process_stdout_cb a wrapper that calls process_process_stdout.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>